NORMAL HJEMATOLOGY 149 



The micrometer is a small piece of glass on which there is a scale 

 marking off equal spaces. This is placed on the diaphragm of the 

 eye-piece of the microscope and put in focus by pushing it up or 

 down as needed. The scale is then compared with a stage micrometer 

 marked in microns and the value of the eye-piece scale thus deter- 

 mined. 



Preparation. Wash the cover-glasses and slides with soap and 

 water and then thoroughly rinse in clean, warm water. Polish the 

 glasses with a clean, soft towel. When handling the slides or cover- 

 glasses hold them always by their edges, and never touch a flat sur- 

 face. Success in preparing fresh-blood specimens depends largely 

 upon the absolute cleanliness of the glasses used. Before using the 

 glasses pass them through the Bunsen or alcohol flame six or eight 

 times while holding them with the fingers, then they will not be 

 broken. Put the glasses down in a clean, safe place, with the heated 

 side up, as this is the side to be used. 



Technique. Obtain the blood as before, using the second or third 

 drop. Bring one of the previously heated cover-glasses underneath 

 the drop of blood and allow it to just touch lightly the center of the 

 glass; then quickly place the cover-glass, blood side down, upon a 

 glass slide. If the glasses are clean, the blood fresh enough, -and of 

 the right amount, the blood will spread out into a thin layer, in which 

 the corpuscles lie oh the flat surface in a single layer. Around the 

 margin the cells will be more or less grouped together. The specimen 

 should then be placed under the microscope and studied at once. 

 Mix a small drop of blood with a small drop of water on a slide and 

 cover with a cover-glass and examine. Smear a drop of blood on a 

 slide by drawing another slide over it, cover one with a glass, and 

 make another and leave uncovered, and examine. 



Precautions. It is very important that the slides and cover- 

 glasses should be kept perfectly clean and dry. If alcohol is used an 

 alcoholic residue is left upon the glass and often interferes with the 

 examination. Touching a glass surface with a freshly cleaned finger 

 will leave enough fat and dirt to prevent the blood spreading. The 

 blood must be transferred to the glasses and covered quickly, or it 

 will partially clot and prevent spreading. The drop must be large 

 enough to give a thin clean field of at least one-half inch in diameter, 

 but it must not be so large that the blood cannot spread out into a thin 

 film between the glasses. The spreading must take place entirely by 

 capillary attraction; pressure must never be used to continue or cause 

 spreading. The glass must touch only the tip of the drop while 

 obtaining the blood; if it touches the ear the blood will not spread. 



Questions. Red Cell. 1. Describe a red cell. 



2. What is the shape of a red cell? 



3. How may the shape of a red cell be demonstrated? 



4. Are the red cells nucleated? 



